Topic Report Absorption & Scattering in epidermis and dermis Reporter: Yen-Chun Liang Advisor: Sheng-Lung Huang 1

Outline Introduction Absorption Scattering Conclusion 2

Introduction 3

Skin structure 4 ~100 μm thick, blood free. 1~4 mm thick, vascularized.

Light-tissue interactions 5 stratum corneum (refractive index) n ~ %~ 7% of incident light is direct reflected by stratum corneum. Remained incident light will be absorbed or scattered.

Absorption 6

Absorption in skin In the UV spectral range, absorption is contributed by protein, nucleic acid,and urocanic acid ( 犬尿酸 ). In the visible spectral range, melanin in epidermis and haemoglobin in dermis define the absorption properties in whole skin. In the NIR region, absorption properties are dominated by lipid in epidermis and water in dermis. 7

Absorption in skin 8 oxyhaemoglobin Water lipid Water + lipid

Absorption in epidermis 9

Epidermis The total optical absorption coefficient of the epidermis depends on a minor baseline skin absorption and a dominant melanin absorption 10

Epidermis absorption 11 Baseline absorption coefficient of melaninless epidermis Absorption coefficient of a single melanosome Volume fraction of melanosomes in epidermis Net epidermal absorption coefficient

Baseline skin absorption 12 (Ruiping Huang, S. Jacqes, data) The above expression is based on measurements of bloodless rat skin using an integrating sphere calibrated with careful phantom measurements

Determining of scattering & absorption coefficients 13 Diffuse reflectance (R) & total transmittance (T) must be measured to determine scattering & absorption coefficients. K:absorption S:scattering

Single melanosome 14 [Ref: Jacques,McAuliffe: Photochem Photobiol (1991)] There is considerable variation in the melanin content of melanosomes so the above expression is only an approximation

Volume fraction of melanosomes 15 "How many melanosomes per unit volume are in the epidermis?"

Net epidermal absorption coefficient 16

Absorption in dermis 17

Dermis The total optical absorption coefficient of the dermis depends on a minor baseline skin absorption and a dominant hemoglobin absorption due to the cutaneous blood perfusion. 18

Dermis absorption 19 Baseline absorption coefficient Absorption coefficient of whole blood Average volume fraction of blood

Absorption coefficient of whole blood 20 (hematocrit 血球容積比 : 定量的血液中含有多少比例的紅血球 ) (ex: 男性： 36 ～ 50% ；女性： 34 ～ 47%)

Scattering 21

Scattering 22 The scattering properties of the whole skin is determined by the reticular dermis Mie scattering (due to large cylindrical dermal collagen fibers) Rayleigh scattering(due to small structure collagen fibers)

Scattering 23

Optical penetration depth of light 24 When penetration depth of light is a priority, the center wavelength of OCT light source may be located around 1000~1130 nm for skin imaging

25 Conclusion

The absorption is described in terms of melanin and hemoglobin absorption proportional to the volume fractions of melanosomes and whole blood. The dermal scattering is described in terms of the relative contributions of Mie and Rayleigh scattering due to collagen fibers. The epidermal scattering, which is affected by its keratin fibers, is sufficiently close to that of dermis and sufficiently thin to not be critical. Skin optics are quite variable in the scattering properties, the degree of melanin pigmentation, and the amount and distribution of blood perfusion. This is only an average skin's optical properties. 26